This invention generally relates to electroplating of metal alloys, and in particular to processes for electrodeposition of nickel and nickel cobalt phosphorous alloys.
The deposition of nickel phosphorous alloys has generally been known in the art. The deposited nickel phosphorous alloys can be useful as corrosion and wear resistance coatings on many different substrates. In addition, they can also be used in decorative coatings and in the fabrication of certain optical components.
Nickel phosphorous alloys can be deposited by electroless or electrolytic processes. However, the electroless processes known in the art are generally limited to the deposition of rather thin nickel phosphorous coatings. This in part is due to the low plating rate, continuous chemical feed, frequent need to remove solution from the tank for maintenance, and high cost associated with these processes. The internal stress in the electroless deposited alloys cannot be precisely controlled in real-time during the plating process and the mechanical properties of the alloys are less than optimum for many operations. For example, electroless processes generally are not suitable for preparing thick deposits or freestanding forms. In addition, the electroless processes typically require a hazardously high plating temperature at or above 85xc2x0 C. and are associated with the evaporation of the plating bath solution forming potentially hazardous vapors.
There has been significant effort in the art in developing and improving electrolytic nickel phosphorous plating processes. For example, U.S. Pat. Nos. 4,673,468 and 4,767,509 disclose that xe2x80x9csulfatexe2x80x9d baths for nickel phosphorous electroplating have relatively poor cathode efficiency and, poor bath conductivity and that unwanted precipitates are easy to form in the bath. The patents disclose that improved alloy quality can be obtained by increasing the anode current density to at least 200 amperes per square foot. The patents propose an all-chloride bath prepared from NiCl2 and H3PO3, H3PO4, NiCO3, Ni(H2PO3)2 and/or HCl. The plating is conducted at a cathode current density of at least 200 amperes per square foot, at a temperature of 78xc2x0 C. or higher, and in an extremely acidic bath having an acid titer in the range of about 9-14 milliliters (9-14 mls of deci-normal sodium hydroxide are required to bring one milliliter of a bath solution to a pH of 4.2.). The plating efficiency is also low resulting in copious hydrogen evolution and high stress in the deposit.
U.S. Pat. No. 4,808,967 provides an electroplating bath consisting essentially of nickel carbonate, phosphoric acid, and phosphorous acid. Sulfate and chloride salts are excluded from the bath. The bath can be used to electroplate circuit board materials containing from 8 to 30 percent by weight of phosphorous. It is stated that because of the lack of chloride and sulfate salts, the plating bath results in circuit board material exhibiting increased stability and decreased porosity.
Despite the efforts in the art, the electrodeposition of nickel phosphorous alloys have generally found limited industrial applications due to the various drawbacks in the heretofore known electroplating processes. For example, plating is generally done at very high current densities, and the plating efficiency is very low. The processes normally require plating at a pH of less than 2.0, making the bath solution very corrosive to base metals. As a result, expensive precious metal anodes such as platinum and rhodium anodes have to be used. The processes typically require plating at high temperatures of above 75xc2x0 C. to increase the cathode current efficiency and to control the internal stress in the deposited alloy. With the high temperature, low plating efficiency, corrosive solution and constant chemical additions required, there has been little incentive to use the prior art electrolytic processes. Accordingly, there remains need for developing improved processes for electroplating nickel phosphorous or nickel cobalt phosphorous alloys.
This invention provides electroplating bath formulations and processes for electrodepositing from the baths nickel phosphorous alloys or nickel cobalt phosphorous alloys that contain at least about 2% and up to 25% by atomic volume of phosphorous. The preferred electroplating bath for electroplating nickel phosphorous alloys has a composition including nickel sulfate, hypophosphorous acid or a salt thereof, boric acid or a salt thereof, a monodentate organic acid or a salt thereof, and a multidentate organic acid or a salt thereof. A surfactant such as Triton X-100 or sodium laurel (dodecal) sulfate is optionally included. For electroplating nickel cobalt phosphorous alloys, the bath contains, in addition, a cobalt source such as cobalt sulfate. The electroplating baths normally have a pH of from about 3.0 to 4.5.
The alloys of the present invention can be electrodeposited from the bath onto a substrate at a current density of less than about 35 mA/cm2 and a temperature of from about 25xc2x0 C. to about 70xc2x0 C., preferably less than about 50xc2x0 C. Anodes such as platinum or other precious metal anodes can be used in the electroplating. Preferably, one or more soluble anodes containing nickel and/or cobalt metal or alloys thereof can be used in electroplating using the electroplating bath of this invention.
In accordance with the electroplating process of this invention, the internal stress in the electrodeposited alloys can be conveniently controlled in real time to zero stress or near zero stress. When electroplating from a specific bath composition of this invention at a given temperature and a predefined pH, the internal stress in an electrodeposited alloy varies with the current density in the electroplating bath. The relationship between the internal stress and the current density, and, in particular, the current density at which the internal stress is zero, can be determined. By monitoring internal stress in the electrodeposited alloy, and adjusting the current density in response to the monitored internal stress, real time control of the internal stress to about zero can be achieved even at a low temperature of less than about 50xc2x0 C.
The electroplating process of this invention may be operated for an extended period of time with little operator intervention required other than simple pH adjustment and occasional additions of phosphorous sources in the electroplating bath. This is in contrast to the constant chemical additions and frequent stripping of the process tanks and equipment required in prior art nickel phosphorous plating processes.
The electroplating process of this invention is normally conducted at a low temperature. As a result, lower energy cost is incurred, and loss of the electroplating bath composition due to evaporation is minimal. In addition, less volatile chemicals evaporate into the air thus alleviating health and safety concerns to a great extent.
The preferred high strength nickel alloys electrodeposited from the bath typically have at least about 8% by atomic volume of phosphorous and generally exhibit exceptional strength and microyield while having a lower density than pure nickel. In a preferred embodiment, the alloy of this invention contains from about 30% to 77% by atomic volume of nickel, from about 15% to 50% by atomic volume of cobalt, and from about 8% to 20% by atomic volume of phosphorus. Typically, the alloys of this invention do not reach 0.2% engineering yield and have a microyield of at least about 86 kg/mm2 (125 ksi), an ultimate strength of at least about 175 kg/mm2 (250 ksi), a density of less than about 8.0 grams/cc and hardness of RC 50 to 54. The alloys can be conveniently machined with hard tools such as high-speed steel, carbides, nitrides or diamond. They are substantially amorphous and can be polished with optical quality abrasives to form excellent quality optical components. Accordingly, the alloys are useful in many industrial applications. In particular, since the alloys have a lower density and high strength, lightweight X-ray mirrors having a large collecting area can be made from them for detecting galactic and extragalactic light sources. The thinner X-ray mirrors can be launched into space at lower cost per unit collection area due to the high microyield strength, preventing permanent deformation.
The foregoing and other advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying examples, which illustrate preferred and exemplary embodiments.